1. Field of the Invention
This invention relates to optical drives used to read data from optical discs, and more particularly, to a focusing and tracking method and system for use on the read/write head of an optical drive to control the focusing and tracking of the same during read operation. This invention utilizes a combination of the knife-edge focusing method and the push-pull tracking method and a specially designed photo detector that can be used to obtain both the focusing error signal and the tracking error signal, allowing the system to be simplified in structural complexity, thereby saving manufacturing cost.
2. Description of Related Art
An optical disc, commonly known as a compact disc (CD), is a mass storage medium, which is a nonmagnetic, polished metal disc that can hold a very larger volume of digital data compared to conventional magnetic discs. In terms of the type and use of the stored data, optical discs are named differently, such as Audio CD, Video CD, CD-ROM (compact-disc read-only-memory) CD-Interactive (CD-I), CD Recordable (CD-R), and CD-Erasable (CD-E). Further, a new type of multiple-layer high-density optical disc named DVD (digital video disc or digital versatile disc) is currently available and gradually becoming very demanding due to its extremely high storage capacity. There exists, therefore, a need for optical drives that are capable of reading data from all of these various types of optical discs.
To read data from an optical disc, the read/write head should be capable of focusing a laser beam on the optical disc. During the read operation, however, external vibrations could cause defocusing to the laser beam. To solve this problem, the optical drive is provided with feedback control means that takes a focusing error signal as feedback signal to thereby control the focusing of the laser beam. Moreover, during read operation, the read/write head is constantly moving radially from track to track so as to seek for the needed data. However, external vibrations or other factors could cause mistracking to the laser beam such that the laser beam is not spotted on the target track where the data to be read out are located. To be able to read data from various types of optical discs, the read/write head should be provided with a laser source capable of generating various wavelengths of laser beams, or alternatively provided with various object lens that can be used to focus the laser beam precisely on the optical disc. In the case of a DVD having a plate thickness of 0.6 mm and a track pitch of 0.74 .mu.m (micrometer), the object lens used in the optical drive for focusing the laser beam should have a numerical aperture (NA) of 0.6 and the laser beam used to read this disc should have a wavelength of 650 nm (nanometer); while in the case of a CD (compact disc) having a plate thickness of 1.2 mm and a track pitch of 1.6 .mu.m, the object lens should have an NA of 0.45 and the laser beam used to read this disc should have a wavelength of 780 nm. Conventional focusing methods for use on an optical read/write head include, for example, the so-called knife-edge focusing method and the astigmatic focusing method. Optical read/write heads that utilize the knife-edge focusing method are disclosed in U.S. Pat. Nos. 4,654,516; 4,701,604; and 4,654,519, to name a few.
FIG. 1 is a schematic diagram showing the architecture of an optical read/write head which utilizes the knife-edge focusing method for reading data from an optical disc, as designated by the reference numeral 1. As shown, this read/write head includes an object lens 2, a knife-edge member 4, a focusing lens 6, and a photo detector 8. The reflected light from the optical disc 1 is first collimated by the object lens 2 into a straight beam. The knife-edge member 4 is disposed in such a manner as to block a half part of the reflected light from the object lens 2, allowing another half part of the same to reach and pass through the focusing lens 6 and subsequently reach the photo detector 8. The spotted area of the reflected light on the photo detector 8 is therefore the half of a circle (i.e., semicircle in shape). The photo detector 8 is formed with two separate light-sensitive areas (not shown), including a bottom light-sensitive area and a top light-sensitive area. In a defocused condition when the reflected light from the optical disc is focused by the focusing lens 6 at a point in back of the photo detector 8, the reflected light will be spotted largely on the top light-sensitive area; whereas when the reflected light from the optical disc is focused by the focusing lens 6 at a point in front of the photo detector 8, the reflected light will be largely spotted on the bottom light-sensitive area; and in focused condition, the reflected light will be spotted on the center of the photo detector 8. The opto-electrical signals generated from the light-sensitive areas of the photo detector 8 are then used to control the read/write head either toward or away from the optical disc 1 until the focusing condition is achieved.
Further, optical read/write heads that utilize the astigmatic focusing method are disclosed in Japanese Patent Sho-59-167863, and Japanese Patent Sho-54-39101, to name a few.
FIG. 2 is a schematic diagram showing the architecture of an optical read/write head which utilizes the astigmatic focusing method for reading data from an optical disc, as designated by the reference numeral 1. As shown, this read/write head includes an object lens 2, a focusing lens group (including a concave lens 6 and a spherical lens 12), and a photo detector 8. The reflected light from the optical disc 1 propagates successively through the object lens 2, the concave lens 6, and the spherical lens 12 to be finally received by the photo detector 8. When the laser beam is in focused condition on the optical disc 1, the reflected light from the optical disc will form a circular spotted area on the photo detector 8. Otherwise, the spotted area will be an elliptical shape. The elliptical shape resulted from a defocusing in front of the optical disc and that resulted from a defocusing in the back of the optical disc are oriented in perpendicular directions on the photo detector 8. In either of these conditions, the light-sensitive areas of the photo detector 8 will receive different amounts of light, thereby generating opto-electrical signals of different magnitudes that can be used to obtain the focusing error signal for feedback control of the focusing of the laser beam used to read data from the optical disc 1.
When reading a high density optical disc, the most widely used tracking method is the so-called push-pull tracking method. The opto-electrical signals obtained from this method are processed by either the differential phase detection (DPD) method, the heterodyne method, or the push-pull method to obtain the tracking error signal. The optical disc is formed with a multiplicity of pits in the surface thereof to represent the data stored thereon. When a laser beam is focused on the optical disc, these pits will act like a grating that causes diffraction to the reflection of the incident laser beam. The constructive interference and destructive interference in the reflected light from the optical disc respectively represent the two values of the binary data stored on the optical disc. The data can be correctly read out only when the laser beam is focused precisely on the target track where the data to be read out are located. When the laser beam is under a mistracking condition, it can be pushed outwards or pulled inwards in the radial direction until the target tracked is spotted.
In conventional read/write heads, the focusing error signal and the tracking error signal are separately detected by different optical systems. Therefore, due to the need to combine the focusing control and tracking control into the same optical system, the read/write head is quite complex in structure and requires a great number of optical components to construct, particularly in the case of constructing a DVD read/write head. The manufacturing cost is therefore quite high. Moreover, conventional DVD read/write heads customarily utilize a combination of the astigmatic focusing method and the push-pull tracking method. In these read/write heads, however, the opto-electrical signals generated from the astigmatic focusing system and those from the push-pull tracking method can cause cross-talk to each other, resulting in a degradation to the accuracy of the focusing and tracking control.